This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Transposable elements are the drivers of genome evolution. Phage Mu is the most well-studied and most efficient transposable element known. The chemistry of transposition occurs within a stable nucleoprotein complex called the 'transpososome'which is built bridging interactions among three DNA sites - the left (L) and right (R) ends of Mu and an enhancer element (E) - mediated by the six subunits of the transposase protein MuA and a dimer of the E. coli protein HU. The DNA follows a well-defined path within the transpososome, trapping five supercoil nodes. The particular arrangement of DNA and protein components lends extraordinary stability to the transpososome and regulates the frequency, precision, directionality and mechanism of transposition. The structure of the transpososome, therefore, holds the key to understanding all of these attributes. and ultimately to explaining the runaway genetic success of transposable elements throughout the biological world.